In hybrid networking of a high frequency base station and a low frequency base station, also referred to as heterogeneous networking (HetNet) of the high frequency base station and the low frequency base station, the low frequency base station covers a relatively large area, and the high frequency base station provides hotspot coverage in the coverage of the low frequency base station, to improve a capacity of a hotspot region. In this networking mode, system overheads of control signaling transmission on a high-frequency directional link can be reduced using a wide coverage capability of the low frequency base station, and therefore this networking mode is a key deployment scenario in future 5G.
In an existing LTE system, Softcell networking is a typical HetNet networking solution. In the Softcell networking, a micro/pico base station (micro/pico BS) reuses a cell ID of a macro base station (macro BS). In other words, the micro base station is transparent for a terminal. When accessing a network, the terminal knows only an ID of an entire HetNet cell or a macro base station ID, and does not need to know a micro base station that serves the terminal. In this networking solution, to implement synchronization between the terminal and the micro base station, the existing technical solution is as follows.
(1) The terminal completes a downlink synchronization process with the macro base station by detecting a primary/secondary synchronization signal (PSS/SSS) of a cell, to obtain a synchronization clock on a network side.
(2) The terminal completes an uplink physical random access process to obtain an uplink timing advance (TA) from the terminal to the macro base station.
(3) Downlink synchronization and an uplink timing advance between the terminal and each micro base station directly reuse a result between the terminal and the macro base station. In the LTE system, a minimum cyclic prefix (CP) length of a symbol is 40.69 μs. It is assumed that a coverage radius of the macro base station is 500 m, a coverage radius of the micro base station is 100 m, a maximum latency difference between the macro base station and the micro base station does not exceed 1.33 μs, and a maximum latency difference obtained when different terminals arrive at the micro base station does not exceed 0.33 μs. Therefore, impact caused by the latency difference can be eliminated using a CP.
In this networking solution, to enable the network side to select a micro base station (for example: a Transmission Point (TP)) or micro base station set that serves the terminal, the existing technical solution is as follows.
(1) The macro base station or a network controller of a HetNet network allocates, to a target terminal, a sending resource carrying an uplink measurement reference signal and another signal.
(2) The terminal sends a specified uplink measurement reference signal using the specified resource.
(3) The network side such as a micro base station adjacent to the terminal in the HetNet performs measurement based on the uplink measurement reference signal, for example, measures reference signal received power (RSRP), to determine the serving micro base station/micro base station set to which the terminal belongs.
However, the foregoing solutions are not applicable to a scenario in which a micro base station is a high frequency base station, because a length of a high frequency symbol is of a microsecond level, for example, 1.25 μs, and a CP length is usually ⅕ of a symbol length, namely, 0.25 μs. Based on previous latency difference estimation, this CP length is not enough to eliminate impact caused by the latency difference between the macro base station and the micro base station and the TA latency difference of different terminals.
FIG. 1 describes the impact brought by the two types of latency differences.
Q1: Although time synchronization and uplink timing advance processes are completed between a terminal and a low frequency macro base station (an eNB in FIG. 1), in other words, a signal sent by a network side clock to the eNB and a signal sent by UE to the eNB in FIG. 1 are aligned in terms of time, when this result is reused on a micro base station, a random latency difference is caused when uplink transmission signals of different terminals (UE 1 and UE 2 in the figure) arrive at a micro base station (TP 1 in the figure). The latency difference damages time domain orthogonality of an uplink measurement reference signal and also interferes with a data part of the uplink measurement reference signal.
Q2: In addition, even for a single terminal (the UE 1 in the figure), when a result that the terminal is aligned with the macro base station in terms of time is reused on the micro base station (the TP 1 in the figure), a latency difference is caused between a low frequency macro base station and a high frequency micro base station because of different path propagation distances. This latency difference causes beam detection missing, and consequently a reference signal sent using a specific beam cannot be completely received.
Therefore, in a HetNet hybrid networking scenario in which a micro base station is a high frequency base station and a macro base station is a low frequency base station, a problem exists that an uplink reference signal cannot be correctly transmitted because synchronization and a TA between a terminal and a macro base station cannot be reused between the terminal and a high frequency micro base station. How to design an uplink reference signal transmission solution to ensure uplink reference signal transmission accuracy is an urgent problem to be resolved.